In this Phase I study, we will test the safety of the drug plerixafor (MOBOZIL) at different dose levels, used together with other anti-cancer drugs—cytarabine and etoposide. We want to find out what effects, good and /or bad, this combination of drugs has on leukemia. Plerixafor is a drug that blocks a receptor on the leukemia cell, which prevents it from staying in the bone marrow where it can be resistant to chemotherapy. Plerixafor is FDA approved for mobilizing stem cells from the bone marrow in preparation for an autologous stem cell transplant. Cytarabine and etoposide have been used as part of standard chemotherapy for ALL and AML. However, the use of plerixafor with cytarabine and etoposide in pediatric patients with relapsed or refractory ALL, AML and MDS is considered experimental.